Polyacetal resin is excellent in the balance of mechanical properties (such as friction resistance, abrasion resistance, creep resistance and dimensional stability), and has extremely excellent fatigue resistance. Furthermore, the resin is excellent in chemical resistance, too, and has a reduced water-absorbing property. Accordingly, while taking advantage of these characteristics, polyacetal resin is used as an engineering plastic for automobile interior parts, house interior parts (hot water mixing plugs, etc.), clothes parts (fasteners, belt buckles, etc.), building material uses (pipes, pump parts, etc.) and mechanical parts (gears, etc.) etc., and shows an increasing demand.
However, polyacetal resin is slightly thermally decomposed by thermal history in the production, molding etc. of the resin. As the result, although in a very minute amount, formaldehyde generates to thereby pollute the molding mold and deteriorate labor (health) environments in molding works. Furthermore, formaldehyde generating from resin products is considered to possibly cause sick house syndrome and the like.
In the circumstances, Ministry of Health, Labour and Welfare announces a guidance value for formaldehyde concentration (the upper limit 0.08 ppm) in building rooms, and thus farther reduction is required of the amount of formaldehyde generation from polyacetal resin molded articles.
It is well-known that the blending of an additive to polyacetal resin may suppress the formaldehyde generation from pellets and molded articles, and, conventionally, various materials have been examined as an additive. For example, there are proposed a melamine-formaldehyde polymer (Patent Document 1), a polyamine reaction product obtained by reacting a reaction product of polyamine and cyanuric chloride with ammonia or a derivative thereof (Patent Document 2), a dicyandiamide compound (Patent Document 3), a silane compound (Patent Document 4), a nitrogen-containing compound-borate (Patent Document 5), a glyoxydiureide compound (Patent Document 6), an urea derivative and/or amidine derivative (Patent Document 7), a condensate of phenols, a basic nitrogen-containing compound and aldehydes (Patent Document 8), a triazine ring-containing spiro compound (Patent Document 9), etc.
Moreover, there is proposed a method for suppressing the formaldehyde generation by producing polyacetal resin using a strong protonic acid such as trifluoromethane sulfonic acid as a catalyst (Patent Document 10). Furthermore, there is also proposed, as the adsorbent for formaldehyde, the use of an aliphatic dihydrazide compound such as an adipic acid dihydrazide or 1,2,3,4-butanetetracarboxylic acid hydrizide compound (Patent Document 11), and the combined use of these nitrogen-containing compounds with a metal salt of fatty acid (Patent Document 12).
For the coloring of polyacetal resin, generally, inorganic pigment or organic pigment is used. But the blending of these pigments tends to increase the formaldehyde generation from polyacetal resin pellets and molded articles thereof. Accordingly, for the polyacetal resin colored with pigment, the suppression of the formaldehyde generation is an extremely important problem.